Giant intrinsic photovoltaic effect in one-dimensional van der Waals grain boundaries

The photovoltaic effect lies at the heart of eco-friendly energy harvesting. However, the conversion efficiency of traditional photovoltaic effect utilizing the built-in electric effect in p-n junctions is restricted by the Shockley-Queisser limit. Alternatively, intrinsic/bulk photovoltaic effect (IPVE/BPVE), a second-order nonlinear optoelectronic effect arising from the broken inversion symmetry of crystalline structure, can overcome this theoretical limit. Here, we uncover giant and robust IPVE in one-dimensional (1D) van der Waals (vdW) grain boundaries (GBs) in a layered semiconductor, ReS2. The IPVE-induced photocurrent densities in vdW GBs are among the highest reported values compared with all kinds of material platforms. Furthermore, the IPVE-induced photocurrent is gate-tunable with a polarization-independent component along the GBs, which is preferred for energy harvesting. The observed IPVE in vdW GBs demonstrates a promising mechanism for emerging optoelectronics applications.

comprehensive understanding of this intriguing BPVE developed at the grain boundary.
3.The manuscript lacks a discussion of the microscopic mechanism underlying the giant BPVE.There is no evidence that the band edge transifion is responsible for the large BPVE.While the reduced recombinafion process can be an important factor, it does not directly create the photocurrent.Thus, a plausible explanafion is required to understand the observed large photocurrent.
To enhance the manuscript's quality and address these concerns, further experiments, discussions, and contextualizafion of results are needed before considering it for publicafion in Nature Communicafions.

Reviewer #3 (Remarks to the Author):
Intrinsic photovoltaic effect reflecfing the symmetry breaking of solids, is now aftracfing much aftenfion due to the potenfial of overcoming the Shockley-Queisser limit in the convenfional solar cells made of semiconducfing p-n juncfions and also its mechanism related with the carrier dynamics or band geometry/topology.In this paper, authors reported the giant bulk photovoltaic effect in one-dimensional ReS2 grain boundary and successfully demonstrated that observed giant photocurrent response precisely reflect the symmetry breaking at the grain boundaries.I think these findings are very interesfing, providing a new design principle of symmetry engineering in two-dimensional materials and resultant funcfional devices.I have several comments and quesfions below.
1. Do authors have any ideas about the origins of the observed intrinsic photovoltaic effect?For example, shift current, which is one mechanism of the photovoltaic effect induced by the linearly polarized light, may show the characterisfic wavelength dependence.Can authors measure it?Also, can authors calculate the shift current based on their calculated band structure?2. Related with the above comments, I would like to know the authors' opinion on why photocurrent is enhanced by applying the posifive gate voltage.3. I am interested in whether we can control (or intenfionally create) the grain boundary or not.Controllability of the grain boundary might be important for making the efficient photovoltaic devices.4. I think edge of the sample also have a similar symmetry breaking as the grain boundary.Does photocurrent appear at the edge of the sample? 5.I am wondering that the word of "intrinsic photovoltaic effect" might be befter than "bulk photovoltaic effect" in the present case because the grain boundary is not the bulk of the sample.1.The generation of photocurrent in transition metal dichalcogenides (TMDs) is known to result from symmetry breaking, a well-established concept.While the grain boundary approach offers a fresh perspective on breaking symmetry, the manuscript does not sufficiently highlight the novelty and significance of this approach in the scientific context.It would be beneficial to emphasize the distinctive aspects of grain boundary-induced symmetry breaking and its potential implications relative to prior research.2.The BPVE is observed using a single photon source; however, the manuscript lacks an exploration of the relationship between photocurrent and light frequency or intensity.Furthermore, the manuscript does not investigate the distinction between the true edge and the grain boundary.The absence of an analysis of light dependence and position dependence on photocurrent generation hinders a comprehensive understanding of this intriguing BPVE developed at the grain boundary.Intrinsic photovoltaic effect reflecting the symmetry breaking of solids, is now attracting much attention due to the potential of overcoming the Shockley-Queisser limit in the conventional solar cells made of semiconducting p-n junctions and also its mechanism related with the carrier dynamics or band geometry/topology.In this paper, authors reported the giant bulk photovoltaic effect in one-dimensional ReS2 grain boundary and successfully demonstrated that observed giant photocurrent response precisely reflect the symmetry breaking at the grain boundaries.I think these findings are very interesting, providing a new design principle of symmetry engineering in two-dimensional materials and resultant functional devices.

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I have several comments and questions below.